Blocked isocyanates

ABSTRACT

A blocked polyisocyanate of the formula 
     
         R--Y.sub.m 
    
     where 
     R is an m valent aliphatic, cycloaliphatic, heterocyclic or aromatic residue; 
     each Y, which may be the same or different, is ##STR1##  where R 1  is, or, when n is more than 1, each R 1 , which may be the same or different, is an alkyl, alkenyl, aralkyl, N-substituted carbamyl, phenyl, NO 2 , halogen or ##STR2##  group where R 2  is a C 1  -C 4  alkyl group; n is 0, 1, 2 or 3; and 
     m is an integer greater than 1 is useful in paint compositions.

This is a continuation of application Ser. No. 07/769,479 filed Oct. 1,1991 (abandoned); which is a continuation of Ser. No. 07/525,713, filedMay 21, 1990 (abandoned); which is a division of Ser. No. 06/892,898,filed Aug. 1, 1986 (now U.S. Pat. No. 4,976,837); which is a division ofSer. No. 06/706,391 (now abandoned), filed Feb. 27, 1985.

The present invention relates to novel blocked polyisocyanates, methodsfor making them, coating and other compositions e.g. paints andelastomers, containing them and methods of electrodeposition of thecoating compositions.

Blocked polyisocyanates are commonly used in paints which also containactive hydrogen containing compounds e.g. amines and alcohols. Certainof these paints can be electrophoretically or conventionally e.g. spraydeposited onto the article to be coated and then subsequently hardenedby heating, often referred to as stoving. During stoving the blockedpolyisocyanates dissociate so that the isocyanate groups becomeavailable to react with the active hydrogen containing compounds leadingto crosslinking and hardening of the paint.

Blocked polyisocyanates reacting with any active hydrogen containingcompound for the purposes of curing by chain extension or crosslinkingare also used in crosslinking acrylic resins for automotive priming andfinishing, formulating one-pack elastomers and surface coatings whichcontain the blocked isocyanate and as a chain extender in a singlestorage stable package which, when cast, can be cured by application oftemperatures above the unblocking temperature.

Blocked polyisocyanates are polyisocyanates in which each isocyanategroup has reacted with a protecting or blocking agent to form aderivative which will dissociate on heating to remove the protecting orblocking agent and release the reactive isocyanate group.

Compounds already known and used as blocking agents for polyisocyanatesinclude aliphatic, cycloaliphatic or aralkyl monohydric alcohols,hydroxylamines and ketoximes.

Currently used blocked polyisocyanates dissociate at temperatures ofaround 160° C. If a blocked polyisocyanate could be used whichdissociated at a lower temperature but was still stable at ambienttemperatures, then heat sensitive materials could be utilised and energysavings could be made. The blocked polyisocyanates of the presentinvention dissociate at a significantly lower temperature than thosecurrently used and are easily made. The presence of a catalyst ispreferred in order to increase the rate of reaction between theliberated polyisocyanate and the active hydrogen containing compound,especially if the active hydrogen group is --OH. The catalyst can be anycatalyst known in the art, e.g. dibutyl tin dilaurate or triethylenediamine.

The present invention comprises a compound of the formula:

    R--Y.sub.m                                                 (I)

where

R is an m valent aliphatic, cycloaliphatic, heterocyclic or aromaticresidue and each Y, which may be the same or different, is ##STR3##where R₁ is, or, when n is more than 1, each R₁, which may be the sameor different, is an alkyl, alkenyl, aralkyl, N-substituted carbamyl,phenyl, NO₂, halogen or ##STR4## group where R₂ is a C₁ -C₄ alkyl group,n is 0, 1, 2 or 3 and

m is an integer >1, preferably 2-6.

When R₁ represents an alkyl or alkenyl group it preferably contains upto 4 carbon atoms. When it is an aralkyl group, it is preferred that thearyl portion is phenyl and that the alkyl portion contains 1 to 4 carbonatoms. When R₁ is a halogen, it is preferably chlorine or bromine.

The blocking agents used in the present invention are pyrazoles of theformula: ##STR5## where R₁ and n are as defined above. Examples of thepyrazoles described include 3,5-dimethylpyrazole, 3-methylpyrazole,4-nitro-3,5-dimethylpyrazole and 4-bromo-3,5-dimethylpyrazole.

The preferred blocking agent is 3,5-dimethylpyrazole.

Some of these pyrazoles can be made by converting acetylacetone (AA)into a derivative which will react with hydrazine to give the desiredpyrazole e.g.

    AA+Na+CH.sub.2 =CHCH.sub.2 Cl→Ac.sub.2 CHCH.sub.2 CH=CH.sub.2

    AA+Na+PhCH.sub.2 Cl→Ac.sub.2 CHCH.sub.2 Ph

    AA+PhNCO→Ac.sub.2 CHCONHPh

The polyisocyanate which is to be blocked may be any organicpolyisocyanate suitable for crosslinking compounds containing activehydrogen e.g. aliphatic including cycloaliphatic, aromatic,heterocyclic, and mixed aliphatic aromatic polyisocyanates containing 2,3 or more isocyanate groups. The group R will normally be a hydrocarbongroup but substitution e.g. by alkoxy groups is possible.

The isocyanate compound may be, for example, ethylene diisocyanate,propylene diisocyanate, tetramethylene diisocyanate, hexamethylenediisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate,2,4,4-trimethylhexamethylene-1,6 diisocyanate, phenylene diisocyanate,tolylene or naphthylene diisocyanate, 4,4'-methylene-bis(phenylisocyanate), 4,4'-ethylene-bis (phenyl isocyanate),ω,ω'-diisocyanato-1,3-dimethyl benzene, ω, ω'-diisocyanato-1,4-dimethylcyclohexane, ω,ω'-diisocyanato-1,4-dimethyl benzene,ω,ω'-diisocyanato-1,3-dimethylcyclohexane, 1-methyl-2,4-diisocyanatocyclohexane, 4,4'-methylene-bis (cyclohexyl isocyanate),3-isocyanato-methyl-3,5,5-trimethyl cyclohexyl isocyanate, dimeracid-diisocyanate, ω,ω'diisocyanato-diethyl benzene,ω,ω'-diisocyanatodimethyl toluene, ω,ω'-diisocyanato-diethyl toluene,fumaric acid-bis (2-isocyanato ethyl) ester ortriphenyl-methane-triisocyanate, 1,4-bis-(2-isocyanato-prop-2yl)benzene, 1,3-bis-(2-isocyanato prop-2yl) benzene, but is preferably freefrom isocyanate groups directly attached to aromatic nuclei.

Use can also be made of polyisocyanates obtained by reaction of anexcess amount of the isocyanate with a) water, b) a lower molecularweight polyol (e.g. m.w.≦300) or c) a medium molecular weight polyol,e.g. a polyol of greater than 300 and less than 8000 m.w., e.g. sucrose,or by the reaction of the isocyanate with itself to give anisocyanurate.

The lower molecular weight polyol comprises, for example,ethyleneglycol, propyleneglycol, 1,3-butylene glycol, neopentyl glycol,2,2,4-trimethyl-1,3-pentane diol, hexamethylene glycol, cyclohexanedimethanol, hydrogenated bisphenol-A, trimethylol propane, trimethylolethane, 1,2,6-hexane triol, glycerine, sorbitol or pentaerythritol.

The di- or poly-isocyanate obtained by the above reaction may have abiuret structure, or an allophanate group.

The blocked polyisocyanate of the formula I is formed by admixing thepolyisocyanate

    R(NCO).sub.m                                               (III)

with a sufficient quantity of a pyrazole of the formula: ##STR6## suchthat the reaction product contains no free isocyanate groups and is aurea of formula I. This reaction is exothermic and since the reactionproduct will dissociate if the temperature is raised sufficiently,cooling may be required to keep the temperature of the reaction mixturedown, preferably to 80° C. or less.

One use of blocked polyisocyanates is in electrophoretically depositedpaints. The invention also comprises a paint composition comprising apigment carrier containing active hydrogen groups, a pigment and acompound of the formula I.

These paints are usually composed of a pigment dispersed in an aqueousdispersion of a resin containing active hydrogen which is to becrosslinked by the polyisocyanate. Preferably the paint contains 0.5 to2 blocked isocyanate groups per active hydrogen containing group.Suitable active hydrogen containing resins include polyamide-polyamineresins, e.g. the product from a dimer fatty acid and an aliphaticpolyamine, carboxylic acid group containing acrylic resins, and tertiaryamine group containing hydroxyacrylic resins and polymers thereof.

The total concentration of the dispersed solids will, of course, dependupon the process for which the paint is to be used. Various standardadditives such as surface active agents, catalysts and anti-oxidants mayalso be incorporated.

The invention also comprises a method of electrodepositing ontosubstrates a paint composition as described above and then heating thedeposited paint to cross link the pigment carrier.

The electrophoretic deposition process is well known and involves theuse of a cathode and an anode in contact with a bath containing thepaint. The surface to be coated is one of the electrodes. On applying avoltage, generally 1 to 3,000 volts, across the electrodes the paint isdeposited over the chosen electrode.

The coated article is removed from the bath and stoved e.g. baked in anoven, in order to release the isocyanate groups which then react withthe active hydrogen in the resin to crosslink and harden the coating.Using coating compositions according to this invention the temperatureto which the coated article must be heated is generally 100° to 140° C.,which is significantly lower than the temperatures required in currentcommercial processes of 160° C. or more.

An added advantage of the process of our invention is the ability toblock polyisocyanates in the presence of alcoholic solvents, becausepyrazoles are much more reactive than alcohols towards polyisocyanates.This also makes it possible to block polyisocyanates at temperatureslower than those used with compounds already known and used as blockingagents for polyisocyanates.

The following Examples illustrate the invention. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE 1

A mixture of 292.4 parts of Trixene L75 (which is a polyfunctionalisocyanate made by the addition of toluene di-isocyanate and a mixtureof trimethylolpropane and diethylene glycol, as a 75% solution in ethylacetate) and 105.8 parts of ethyl acetate is stirred while 94.9 parts of3,5-dimethylpyrazole is added during 5 minutes, the temperature risingfrom 23° C. to 55° C. After 23/4 hours the temperature falls to 27° C.The I.R. Spectrum shows NCO absent.

EXAMPLE 2

A mixture of 91.5 parts of trimethylolpropane, 273.2 parts of propylenecarbonate and 454.9 parts of isophorone diisocyanate is stirred for 21/2hours at 70° C. and then held at 55° C. for 16 hours. The product thusformed contains 10.37% NCO. 809.3 parts of this product is stirred at33° C. while 201.0 parts of 3,5-dimethylpyrazole is added, followed by470.8 parts of the monomethyl ether of propylene glycol (Dowanol PM).The temperature rises to 52° C. and a clear solution is formed. The IRspectrum shows NCO absent. Analysis shows only 0.5% of freedimethylpyrazole to be present.

EXAMPLE 3

A mixture of 861 parts of isophorone diisocyanate, 535.95 parts ofpropylene carbonate and 2.15 parts of Dabco TMR* is stirred andgradually heated. When the temperature reaches 55° C. heating isdiscontinued. The exothermic reaction raises the temperature to 131° C.in 15 minutes. After cooling to 89° C., 5.8 parts of a 10% solution ofDabco TMR* in propylene carbonate is added. The temperature rises to 90°C. in 8 minutes. After heating to 132° C. it is allowed to cool. 1397.1parts of this product, the tri-isocyanatoisocyanurate formed from 3moles of isophorone diisocyanate as a 61.4% solids solution in propylenecarbonate, having an NCO content of 10.94% is stirred while 366.8 partsof 3,5-dimethylpyrazole is added, cooling as required, to keep thetemperature below 80° C. followed by 463.4 parts of the monomethyl etherof propylene glycol. The mixture is stirred at 50° C. and a clearsolution formed which solution is then allowed to cool. The IR Spectrumshows NCO absent.

EXAMPLE 4

800 parts of pre-polymer from polypropylene glycol of average molecularweight 1000 and 80:20 2,4:2,6-toluene diisocyanate, containing 5.32% NCOand which has been substantially freed from free toluene diisocyanate bythin film evaporation, is stirred while 102.14 parts of3,5-dimethylpyrazole is added. After stirring for 11/4 hours thetemperature rises from 25° to 46°. The temperature is held at 46° for afurther 11/4 hours then raised to 80° during 40 minutes and held at 80°for 110 minutes. The product is a clear pale amber-coloured liquid.

EXAMPLE 5

The preparation referred to in Example 2 above using3,5-dimethylpyrazole (3,5-DMP) as the blocking agent, was repeated butmethyl ethyl ketoxime (MEKO) was used as the blocking agent. Theproducts of each of these preparations was then mixed with thestoichiometric quantity of 1,4-butanediol and 1% dibutyl tin dilauratewas added and then the mixture was coated onto steel panels. Thecoatings were allowed to dry at room temperature for 5 days and thenstoved in an oven at the specified temperature for 30 minutes. Thepanels were then tested for pencil hardness as an indication of cure.

    ______________________________________                                        Temperature                                                                   in °C.                                                                          MEKO Blocked    3,5-DMP Blocked                                      ______________________________________                                        100      Fails HB, no cohesive                                                                         Fails HB, no cohesive                                         strength        strength                                             120      Fails HB, no cohesive                                                                         Passes 5H, cohesive                                  2        strength        film                                                 135      Fails HB, no cohesive                                                                         Passes 7H, cohesive                                           strength        film                                                 160      Passes 7H, cohesive                                                                           Passes 7H, cohesive                                           film            film                                                 ______________________________________                                    

The above results show an almost 40° C. improvement in cure temperaturewith the 3,5-dimethylpyrazole blocked polyisocyanate.

EXAMPLE 6

A paint base is made from the following:

    ______________________________________                                                                 parts                                                ______________________________________                                        1.    Xylene                   7.90                                           2.    Titanium Dioxide RTC60 *(1)                                                                            38.27                                          3.    Additive T.I. *(2)       1.60                                           4.    10% Acronal 700L Solution in Xylene *(3)                                                               0.40                                           5.    Multiflow *(4)           0.13                                           6.    Synocure 867S *(5)       51.70                                          ______________________________________                                         *(1) RTC60  Tioxide U.K. Ltd                                                  *(2) Additive T.I.  Bayer U.K. Ltd                                            *(3) Acronal 700L  BASF U.K. Ltd                                              *(4) Multiflow  Monsanto PLC                                                  *(5) Synocure 867S  Cray Valley Products Ltd                             

Method

Charge the Xylene, Additive T.I., 10% Acronal Solution and Multiflow,gradually adding the Titanium Dioxide and half of the Synocure to givesufficient wetting for efficient grinding. Grind under high shear toHegmann 5, then add the rest of the Synocure.

A mixture of 58.0 parts of the above described paint base, 22.2 parts ofthe product from Example 2 and 0.4 part of dibutyltin dilaurate wascoated on two steel panels and allowed to dry for seven days at ambienttemperature. One panel was stoved for 1/2 hour at 120° C., and the otherfor 3/4 hour at 100° C. Tests for pencil hardness showed that bothpassed 5 H.

Similar results (5 H pencil hardness) were obtained using equivalentamounts of the products prepared as described in Example 2 but usinginstead of 3,5-dimethylpyrazole the equivalent amounts of4-nitro-3,5-dimethylpyrazole, 4-benzyl-3,5-dimethylpyrazole, methyl5-methylpyrazole-3-carboxylate, 4-bromo-3,5-dimethylpyrazole, pyrazole,3-methyl-5-phenylpyrazole and 3,5-dimethylpyrazole-4-carboxanilide(prepared by condensation of hydrazine acetate with diacetoacetanilide).A similar result was also obtained using a product prepared as describedin Example 2 but using Pentoxone* (4-methoxy-4-methylpentan-2-one)instead of propylene carbonate. A panel coated with the paint base andstoved for 1 hour at 120° with no crosslinker present failed an HBpencil test.

We claim:
 1. A blocked polyisocyanate of the formula:

    R--Y.sub.m

wherein m is an integer greater than 1; R is an m valent,cycloaliphatic, heterocyclic or aromatic residue; each Y, which may bethe same of different, is ##STR8## in which n is 0, 1, 2 or 3: when n is1, R¹ is alkyl, alkenyl, aralkyl, N-substituted carbamyl, phenyl, NO₂,halogen or --CO--O--R², and R² is C₁ -C₄ alkyl;when n is more than 1,the groups R¹ may be the same or different and each is alkyl, alkenyl,aralkyl, N-substituted carbamyl, phenyl, NO₂, halogen and --CO--O--R²where R² is C₁ -C₄ alkyl;provided that, when R is an aromatic residue,the groups Y are not directly attached to an aromatic nucleus.
 2. Ablocked polyisocyanate according to claim 1 wherein at least one Y is##STR9##
 3. A blocked polyisocyanate according to claim 2 wherein atleast one Y is ##STR10##
 4. A blocked polyisocyanate according to claim1 wherein m is from 2 to
 6. 5. A blocked polyisocyanate according toclaim 1 wherein R is a hydrocarbon group optionally substituted by analkoxy group.
 6. A blocked polyisocyanate according to claim 1 wherein Ris a divalent radical derived from isophorone of formula ##STR11##
 7. Ablocked polyisocyanate according to claim 1 wherein. R is an aromaticresidue of an isocyanate compound selected from the group consistingofω, ω'-diisocyanato-1,3-dimethyl benzene, ω,ω'-diisocyanato-1,4-dimethyl benzene, ω, ω'-diisocyanato-diethylbenzene, ω, ω'-diisocyanato-dimethyl toluene, ω, ω'-diisocyanato-diethyltoluene,
 1. 3-bis-(2-isocyanato-prop-2-yl) benzene,and1,4-bis-(2-isocyanato-prop-2-yl) benzene.
 8. A blocked polyisocyanateaccording to claim 7 wherein R is the residue of1,3-bis-(2-isocyanato-prop-2-yl) benzene.
 9. A blocked polyisocyanateaccording to claim 1 which has a biuret structure or an allophanategroup.
 10. A blocked polyisocyanate according to claim 1 wherein R is aresidue of a polyisocyanate reaction product of an isocyanate and anactive hydrogen-containing compound selected from the group consistingof water, a lower molecular weight polyol having a molecular weight lessthan or equal to 300 and a medium molecular weight polyol having amolecular weight greater than 300 and less than
 8000. 11. A blockedpolyisocyanate according to claim 10 wherein said activehydrogen-containing compound is a lower molecular weight polyol selectedfrom the group consisting of ethylene glycol, propylene glycol,1,3-butylene glycol, neopentyl glycol, 2,2,4-trimethyl-1,3-pentane diol,hexamethylene glycol, cyclohexane dimethanol, hydrogenated bisphenol-A,trimethylol propane, trimethylol ethane, 1,2,6-hexane triol, glycerine,sorbitol and pentaerythritol.
 12. A blocked polyisocyanate according toclaim 1 wherein R is a residue of an isocyanurate.
 13. A method ofcoating a substrate which method comprises applying to the substrate acomposition comprising an active hydrogen-containing compund and ablocked polyisocyanate and thereafter heating said blockedpolyisocyanate at from 100° to 120° C., said blocked polyisocyanatebeing a compound of the formula

    R--Y.sub.m

wherein m is an integer greater than 1; R is an m valent aliphatic,cycloaliphatic, heterocyclic or aromatic residue; each Y, which may bethe same or different, is ##STR12## in which n is 0, 1, 2 and 3; when nis 1, R¹ is alkyl, alkenyl, aralkyl, N-substituted carbamyl, phenyl,NO₂, halogen or --CO--O--R², and R² is C₁ -C₄ alkyl;when n is more than1, the groups R¹ may be the same or different and each is alkyl,alkenyl, aralkyl, N-substituted carbamyl, phenyl, NO₂, halogen and--CO--O--R² and, R² is C₁ -C₄ alkyl;provided that, when R is an aromaticresidue, the groups Y are not directly attached to an aromatic nucleus.14. A method according to claim 13 which comprises electrodepositingsaid composition on said substrate.
 15. A method according to claim 13wherein said composition is a paint comprising a pigment carriercontaining active hydrogen groups, a pigment and said blockedpolyisocyanate.
 16. A method according to claim 15 which compriseselectrodepositing onto said substrate said paint composition and thenheating the deposited paint to cross link said pigment carrier.
 17. Aprocess for producing the blocked polyisocyanate as defined in claim 1which comprises admixing a polyisocyanate of formula

    R(NCO).sub.m

with a quantity of a pyrazole of formula ##STR13## where R, m and n areas defined in claim 1 such that the blocked polyisocyanate contains nofree isocyanate groups.